Thin-film deposition using laser ablation : application to ferromagnetic shape-memory materials and methods for spatial shaping of laser beams

摘要

Ferromagnetic shape-memory (FSM) compounds, in particular Ni-Mn-Ga alloys, are actively studied materials due to their large, up to 10 %, shape changes in an external magnetic field. This makes it possible to realize novel actuators and sensors. So far, the research on Ni-Mn-Ga has focused on its bulk properties but for the fabrication of micromechanical components and even microscopic machines, the material should be in thin-film form. Pulsed laser deposition (PLD) is an effective technique to deposit high-quality thin films of various multicomponent materials such as Ni-Mn-Ga. PLD is based on laser ablation where high-power laser pulses are applied to stoichiometrically transfer material from a target surface onto a substrate. In this thesis, Ni-Mn-Ga films with a thickness of a few hundred nanometers were deposited on different substrates such as silicon, NaCl and small pieces of bulk Ni-Mn-Ga. The deposition parameters such as the substrate temperature and the laser-pulse energy density on the target were optimized in order that the films would be ferromagnetic and would have the desired crystal structure and a smooth surface. In addition, methods were developed to release the film from its substrate such that the necessary phase transformation for the occurrence of the shape-memory effect would be observed at low temperatures. Particularly promising results were obtained by using NaCl substrates: the magnetization was almost 70 % of the typical bulk value, the substrate could be easily removed by isotropic etching with water, and signs of the phase transformation were observed at around −100 °C. Also films on silicon were highly ferromagnetic, and when deposited using the optimal parameters, the number density of droplets on their surface was low. The results indicate that PLD combined with microlithography is a competitive technology that can be applied to realize thin-film FSM structures.For efficient ablation, the intensity distribution of the laser beam on the target should be smooth with steep edges. In this work, such a distribution was realized with the help of diffractive optics: periodic diffractive elements transformed the beam coming out from our excimer laser to the desired flat-top distribution on the focal plane of a positive lens. This new way of shaping laser beams reduces the power losses of our PLD system. Diffractive optics was also applied to directly produce almost propagation-invariant laser beams, so-called Bessel-Gauss beams in laser resonators. Furthermore, Bessel-like laser beams were created as a result of self-focusing in a certain liquid crystal. Such diffraction-free laser beams can be used in precise laser ablation and accurate patterning of small structures in thin films.